The invention provides a process for preparing aromatic aldehydes which possess at least one hydroxyl group on the aromatic ring.
One example of a literature procedure for preparation of hydroxy-substituted aromatic aldehydes is the O-demethylation of aromatic methyl ethers. However, the industrial scale use of known methods for O-demethylation is limited since, for example, the reagents used are expensive, promote corrosion and/or require very severe reaction conditions, for example reaction by means of sodium thioethoxide at 150° C. in dimethylformamide or reaction with sodium in liquid ammonia. Reactions with highly reactive reagents, for example boron tribromide, aluminum trichloride or boron trichloride, do lead to a rapid conversion but exhibit a significantly reduced selectivity as soon as two or three aromatic methoxy groups are present on the aromatic ring. The use of strong Lewis acids or Lewis bases also becomes difficult when the aromatic comprises further functional groups, for example aldehyde, keto or benzylic alcohol groups, since side reactions can occur more frequently in this arrangement.
The regioselective O-demethylation of poly-methoxy-substituted benzaldehydes is known from the literature, for example:    Demyttenaere et al., Tetrahedron 2002, 58, 2163-2166, uses AlCl3 for demethylation of 2,3,4-trimethoxybenzaldehyde. The demethylation is effected in the presence of a large excess of AlCl3 in the ortho and para positions.    Ren et al., Tetrahedron Asymm. 2002, 13, 1799-1804, uses a piperidine-water mixture for demethylation of 2,3,4-trimethoxybenzaldehyde. Disadvantages are the long reaction times and the slightly reduced applicability of this reaction to only highly electron-rich compounds.    Bhattacharya et al., Tetrahedron Lett. 2006, 565-567, uses NaSCN and Triton-X 405 for demethylation of 3,4-dimethoxybenzaldehyde. This conversion requires relatively high temperatures.    Fang et al., J. Mol. Cat. A: Chemical 2007, 16-23, uses LiCl in dimethylformamide under microwave irradiation for demethylation of 2,3,4-trimethoxybenzaldehyde and 3,4-dimethoxybenzaldehyde. The conversion is performed in the presence of expensive LiCl. The conversion also forms toxic methyl chloride.    Prager and Tan, Tetrahedron Lett. 1967, 38, 3661-3664, use Lewis acids, for example AlCl3, for demethylation of 3,4-dimethoxybenzaldehyde. A disadvantage in this procedure is the corrosion problems with this reagent, and the requirement for exact setting of the molar ratio.    Pearl et al., J. Am. Chem. Soc. 1952, 74, 4262-4263, uses sulfuric acid for demethylation of 2,3,4-trimethoxybenzaldehyde. Disadvantages are the corrosion problems with this reagent, and, when this procedure is applied to 3,4-dimethoxybenzaldehyde, a conversion in reverse regioselectivity compared to 2,3,4-trimethoxybenzaldehyde.